1. Field of the Invention
The present invention relates to an organic electroluminescence device (hereinafter abbreviated as an organic EL device) and a material for an organic electroluminescence device. Particularly, the present invention relates to an organic electroluminescence device including an emitting layer for emitting red light and a material for the organic electroluminescence device.
2. Description of Related Art
A known organic EL device includes an organic thin-film layer between an anode and a cathode, the organic thin-film layer including an emitting layer, and such an organic EL device emits light using exciton energy produced by recombination of holes with electrons.
Such an organic EL device is expected to be applied as a light-emitting device that is excellent in luminous efficiency, image quality, power consumption and a design property of a thin size, making the best use of the advantages as a self-emitting device.
Points for further improving an organic EL device include, for example, luminous efficiency.
In this regard, development of emitting materials (phosphorescent materials) that emit light using a triplet exciton is promoted in order to enhance internal quantum efficiency. In recent years, a phosphorescent organic EL device has been reported.
Internal quantum efficiency can be enhanced up to 75% or more (up to approximately 100% in theory) by using the above phosphorescent materials to form an emitting layer (phosphorescent emitting layer), and an organic EL device having high efficiency and consuming low power can be obtained.
Further, a known method for forming an emitting layer is a doping method, according to which an emitting material is doped as a dopant into a host material.
In an emitting layer formed by the doping method, excitons can efficiently be produced from charges injected into the host material. Further, exciton energy of the produced excitons is transferred to the dopant, so that light can be emitted from the dopant with high efficiency.
In this respect, in order to intermolecularly transfer energy from a host material to a phosphorescent dopant, excited triplet energy EgH of the host material has to be larger than excited triplet energy EgD of the phosphorescent dopant.
CBP (4,4′-bis(N-carbazolyl)biphenyl) is known as a representative example of a material having effectively large excited triplet energy (e.g., see Document 1: US 2002/182441).
Use of CBP for a host material makes it possible to transfer energy to a phosphorescent dopant that emits light of a predetermined wavelength (for example, green and red). With this arrangement, an organic EL device having a high efficiency can be obtained.
When CBP is used as a host material, luminous efficiency is markedly enhanced because of phosphorescence emission. On the other hand, however, the life thereof is so short as to be not suitable for practical use.
The above problem is considered to be attributable to heavy degradation of molecules by holes due to a not-high oxidation stability that the molecular structure of CBP exhibits.
Further, Document 2 (WO 2005/112519) discloses a technique according to which a condensed-ring derivative having a nitrogen-containing ring such as carbazole is used as a host material for a red-phosphorescent emitting layer. Luminous efficiency and the lifetime are improved by the above technique, but it is not satisfactory in a certain case in putting into practical use.
On the other hand, various kinds of host materials (fluorescent hosts) for fluorescent dopants are known, and various proposals have been made on host materials capable of forming a fluorescent emitting layer which is excellent in luminous efficiency and lifetime by combination thereof with fluorescent dopants.
Excited singlet energy Eg(S) of a fluorescent host is larger than that of a fluorescent dopant, but excited triplet energy Eg(T) thereof is not necessarily larger than that of the fluorescent dopant. Accordingly, the fluorescent host cannot simply be applied as a host material (phosphorescent host) for a phosphorescent emitting layer.
For example, anthracene derivatives are well known as a fluorescent host.
However, anthracene derivatives have relatively-small excited triplet energy Eg(T) of approximately 1.9 eV. Accordingly, energy cannot be reliably transferred to a phosphorescent dopant for emitting light having a wavelength in a visible light region of 520 to 720 nm. Further, excited triplet energy cannot be trapped within a emitting layer. Accordingly, anthracene derivatives are unsuitable for a phosphorescent host.
Further, perylene derivatives, pyrene derivatives and naphthacene derivatives are also not preferred for a phosphorescent host for the same reasons.
An example in which aromatic hydrocarbon compounds are used for a phosphorescent host is known (e.g., see Document 3: JP-A-2003-142267). In this example, a compound in which two aromatic groups are bonded to a benzene central skeleton as substituents in meta positions is used for a phosphorescent host.
However, the aromatic hydrocarbon compounds described in Document 3 structured such that the molecules are extended in a manner symmetric relative to the benzene central skeleton, the emitting layer can be easily crystallized.
On the other hand, organic EL devices in which various aromatic hydrocarbon compounds are used are disclosed in Document 4 (WO 2007/046685), Document 5 (JP-A-2006-151966), Document 6 (JP-A-2005-8588), Document 7 (JP-A-2005-19219), Document 8 (JP-A-2005-197262) and Document 9 (JP-A-2004-75567). However, effectiveness thereof as the phosphorescent host is not referred at all.
As described above, a host material that can efficiently transfer energy to a phosphorescent material and has a practically long lifetime has not yet been known, which has hindered a practical realization of a device in which a phosphorescent material is used.